The present invention relates to a method for forming an ultrasonic image in a non-destructive ultrasonic scanning inspection of a body to detect a defect inside thereof. More particularly, the invention relates to a method for forming an ultrasonic image having increased sharpness with improved resolution.
The ultrasonic imaging method is widely practiced in many fields including the medical technology, in which a direct-contact probe is used, with an object to visually detect any defect or abnormality in the body of a patient at a desired depth from the skin surface. The ultrasonic imaging method is performed by scanning a beam of ultrasonic waves in place of light or X-rays to form an image. Like other imaging methods, the resolution of images formed by the ultrasonic imaging method is improved as the frequency of the ultrasonic waves is increased although this improvement in the resolution is obtained at the sacrifice of penetration of the ultrasonic beams into the body as a consequence of the increase in the scattering and absorption of the ultrasonic energy. For example, so called ultrasonic microscopes, which utilize ultrasonic waves having an extremely high frequency of 1 GHz or higher, can be used only for the inspection of the very surface layer of the body having a thickness of as small as a few .mu.m due to absorption of the ultrasonic waves by the body although the ultrasonic image of the surface layer obtained with an ultrasonic microscope has a resolution comparable to that obtained with an optical microscope.
When the frequency of the ultrasonic waves is decreased to several MHz as in the conventional ultrasonic inspection method, the problem of the ultrasonic attenuation is no longer a limiting factor of the method but the resolution of the image is decreased so greatly that details of the inner structure of the body can hardly be visually inspected.
In most of ceramic materials, composite materials, integrated circuits and the like as an objective of the ultrasonic inspection method, on the other hand, it is sufficient in most cases that the ultrasonic beams reach a depth of a few millimeters from the surface so that ultrasonic waves having a frequency of several tens of MHz are satisfactory in respect of the balance between penetrability of the beams and resolution of the image.
Thus, it is important and eagerly desired to develop a method for improving the resolution of the ultrasonic images in the ultrasonic inspection method without sacrifice in the penetrability of the ultrasonic beams in the frequency region from a few MHz to several tens of MHz.